1. Field of the Invention
The present invention relates to a thermal ink-jet printhead with an improved heater arrangement and a method of generating homogeneous or spontaneous nucleation using the heater arrangement.
2. Description of the Prior Art
Ink-jet printing technologies have been developed and there are several printers on the market which successfully employ the sudden growth of a vapor bubble to eject a minute droplet of ink toward a sheet of paper or the like. Ink-jet recording features inherently quiet printing in that nothing strikes a paper except the ink.
One of conventional ink-jet recording technologies is disclosed in Japanese Patent Application No. 53-101189 which was published for opposition purposes on Dec. 18, 1986 under publication No. 61-59914. The above-mentioned Japanese Patent Application was filed in the United States claiming Convention Priority under U.S. Ser. No. 827,489, which was granted Feb. 2, 1988 and assigned U.S. Pat. No. 4,723,129. This known technique is characterized by a multi-orifice ink-jet printhead with a simple structure and a high-speed recording on a plain paper.
Before turning to the present invention it is deemed advantageous to briefly discuss a known printhead arrangement which is disclosed in the above-mentioned Japanese patent application with reference to FIGS. 1 to 4.
FIG. 1 is a partially sectioned view showing an internal structure of an ink-jet printhead 10. A heat-generating resistor or heating element 12 is provided on a heat accumulating layer 14 which has been evenly deposited on a substrate (viz., base plate) 16 through the use of evaporation, plating or the like technique. Electrodes 18, 20 are coupled to the heating element 12 and apply electrical currents thereto. The heating element 12 and the electrodes 18, 20 are covered with a protective layer 22. According to the description in the above-mentioned U.S. Pat. No. 4,723,129, the protective layer 22 is to prevent electric leaking from one of the electrodes (18 or 20) to the other through a liquid 24 and/or to prevent the elements 12, 18 and 20 from being contaminated by the liquid 24.
An ink supply chamber 26 is formed by a cover plate 28, a chamber lid 30 and the substrate 16. The ink supply chamber 26 communicates with each of a plurality of nozzles (only one is shown in the drawing and is designated by reference numeral 32) which is defined between the substrate 16 and the cover plate 28. Each of the nozzles communicates with an ink supply pipe (not shown).
The aforesaid prior art makes the use of film boiling for ejecting a droplet of ink, the thermal excitation mechanism of which has been discussed with reference to a boiling curve shown in FIG. 2.
In FIG. 2, "dT" on the abscissa indicates a temperature difference between a surface temperature Tr of the heating element 12 and a boiling temperature Tb of the liquid, while a heat flux "Et" transferred from the heating element 12 to the liquid 24 is represented by the ordinate. The boiling curve shows that sudden boiling is induced when the temperature difference dT exceeds a region A-B. Nuclei boiling occurs in a region B-C-D while film boiling takes place in a region E-F-G. As mentioned above, the prior art makes the use of film boiling, which occurs at a point E, by heating the liquid in the vicinity of the heating element 12 in the order of A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E. When the film boiling occurs, a film vapor is induced on the surface of the heating element 12 and prevents the heat transfer from the heating element 12 to the liquid surrounding the film vapor. Thus, the film vapor is volumetrically decreased due to adiabatic phenomenon and is forced to collapse at a high speed.
FIG. 3 is a close-up sectional view of the heating element 12 and the vicinities thereof of FIG. 2, while FIG. 4 is a sectional view taken along a section line X-X' of FIG. 3. The dimensions of each of the members of FIG. 3 are not precisely shown and, the thickness of the heating element 12 is in fact ten to fifty times that of each of the electrodes 18, 20. Such a large difference in thickness tends to cause undesirable cracks in the contacting portions 40 between the resistor 12 and the electrodes 18 and 20, due to the thermal stresses caused by repeated cycle of heating and cooling of the resistor (heating element) 12. More specifically, such undesirable cracks are caused by the differences of coefficients of linear expansions of the members 12, 18 and 20.
As referred to above, the prior art utilizes film boiling for ejecting a droplet of ink by heating along with the points A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E. This causes separation of ink from the heating surface at the point E, and thus the heat flux transition efficiency to the liquid abruptly drops. Accordingly, the surface temperature of the heating element 12 rises abruptly and hence a so-called dry-out phenomenon is induced on the surface of the heating element 12. Therefore, the prior art has encountered the drawback in that the heating element 12 is degraded due to the dry-out phenomena.
Film boiling is caused by heterogeneous nucleation due to very minute gas bubbles formed on the heater surface irregularities (scratches, fine cavities, for example). These gas bubbles are called nucleation sites. The heterogeneous nuclei are observed at an early stage of heating and grow relatively slowly and, accordingly, the prior art is inherently suffered from the difficulty that heat flux transition from the heater surface to an activated liquid layer formed just thereabove is insufficient.